In the past, permanent or biodegradable devices have been developed for implantation within a body passageway to maintain vascular patency. These devices are typically characterized by the ability of such an intravascular device to be enlarged radially after having been introduced percutaneously, to be transported transluminally, and to be positioned in a desired location. These devices are either expanded mechanically, such as by the expansion of a mandrel positioned inside the device, or are capable of releasing stored energy to expand themselves upon actuation within the body.
U.S. Pat. Nos. 4,739,762, 4,776,337 and 4,733,665 disclose expandable and deformable intraluminal vascular grafts in the form of thin-walled tubular members which are expanded radially outwardly into contact with a body passageway, the members being plastically deformed beyond their elastic limit and the members being permanently fixed within the body. Suitable materials for the fabrication of these tubular-shaped members would include silver, tantalum, stainless steel, gold, titanium, or other suitable plastically deformable materials which may be permanently deformed. Permanent deformation is achieved when the material is subjected to a force which creates a strain greater than the elastic limit of the material which is utilized to make the tubular member. The open-mesh configuration of such devices is soon encapsulated by body tissue and cannot be removed. The exceeding of the elastic limit of the material used in such devices is also believed to compromise the performance of the devices in situ.
U.S. Pat. No. 4,969,458 discloses a vascular stent formed from a wire component made of material, such as copper alloy, titanium, or gold, wherein the wound configuration unwinds upon expansion and becomes a permanent prosthesis stent, similar to prior art devices disclosed above, and is not removable.
U.S. Pat. No. 4,969,890 discloses various configurations of shape-memory alloy members which have been previously radially compressed and which, upon positioning within the body and thermal activation, expand by themselves to become a permanent prosthesis within the body. In this regard, the reference teaches a device which operates in a similar fashion to the device disclosed in U.S. Pat. No. 4,485,816. U.S. Pat. No. 4,485,816 discloses a shape-memory alloy staple which, when heated, penetrates and cinches tissue together. Shape-memory alloy historically has been used to perform work in such a fashion wherein the component remains in a strong austenitic state after temperature activation. That is, above its transition temperature from marten site to austenite, and as the references above disclose, the shape-memory alloy either dilates an incompetent blood vessel or holds segments of tissue together. Neither of these devices is practically removable by a method which does not require surgery.
Shape-memory alloys possess the useful characteristic of being capable of changing physical dimensions upon heating above a first transition temperature, A.sub.f, between a soft martensitic metallurgical state and a hard austenitic metallurgical state of the alloys. A shape-memory alloy member can be processed while in a high temperature austenitic phase to take on a first configuration. After cooling the shape-memory alloy member below a second transition temperature M.sub.f between the austenitic and martensitic states without change of physical dimensions, the shape-memory alloy member can be mechanically deformed into a second configuration. The shape-memory alloy member will remain in this second configuration until further heating to a temperature above A.sub.f at which time the shape-memory alloy member will revert to its first configuration. A shape-memory alloy member can exert large forces on adjacent members during the transition from the second configuration to the first configuration. Numerous inventions have taken advantage of shape-memory alloy members capable of exerting this thermally activated force.
Shape-memory alloys have the further useful characteristic that, in the martensitic phase, the stress-strain curve exhibits a plateau indicating that a limited increase in strain can be achieved with imperceptible increase in stress. This martensitic stress-strain plateau usually defines the range of mechanical strain which can be recovered by the application of heat. Exceeding the upper end of this strain range may result in non-heat recoverable deformation.
U.S. Pat. No. 5,197,978, hereby incorporated by reference, discloses shape-memory alloy tissue supporting devices that are made to expand or shrink radially upon mechanical or thermal actuation, and, in particular, devices that are removable from the body.
It would be advantageous to have a tissue supporting device of a generally tubular configuration which can be inserted into a body duct or cavity while in an unexpanded shape and then be expanded to provide permanent support for the tissue forming the duct or cavity, such that the device when expanded does not exert a radial load on the supported duct or cavity and where the device when expanded has sufficient crush resistance to provide support for the duct or cavity when the duct or cavity exerts a normal radial compressive load on the device as the result of major contractions of the tissue.
It would be further advantageous to have a tissue supporting device, for simultaneous support of cavities of different sizes, in which larger expanded device sizes do not require higher expansion pressures than smaller device sizes, so that the potential for dissection and/or tissue damage is minimized, and where further the device remains somewhat flexible to accommodate movement of soft tissue.
It would be further advantageous to have a heat-to-expand tissue supporting device that does not need to be cooled prior to installation and which provides permanent tissue support while in the martensite state during service.
It would be further advantageous to have a method for reversibly manipulating the configuration of a device designed for tissue support, in order to facilitate machining, deburring, etc. of hard-to-reach interior surfaces of the device without affecting the functionality of the device in a final product.